The present invention relates to a process for synthesizing engineering materials using intermittent combustion, or pulse detonation, processes, the engineering materials formed by those processes, and the apparatus for making them. The present invention has particular utility in the synthesis of ceramic materials. It has long been recognized that ceramic materials possess certain properties which are superior to other "engineering materials". They may be tailored for specific applications requiring outstanding heat transfer, resistance to thermal shock, corrosion resistance, wear resistance, specific optical properties, etc.
Conventional ceramic processing techniques usually involve several steps. First, raw materials must be purified into basic ceramic materials. Second, the product is fabricated into the desired shape or form. Third, the product undergoes an initial drying phase. Fourth, the part must be heated to allow the microstructure of the material to mature into its final form. This step is also called sintering. Depending on the specific requirements of the end product, additional processing or finishing steps may be necessary.
Most base ceramic powders arc manufactured using chemical techniques that require several intermediate steps and are fairly slow. Examples arc the sol-gel, solvent evaporation, and various precipitation techniques. These processes tend to yield small amounts of relatively inconsistent powders that typically require further processing.
Most prior processes produce relatively coarse particles. The ideal particles for dense, strong microstructures are fine and spherical. Fine powders on the order of 1 .mu.m typically are produced using mechanical means such as ball milling. The ceramic powder is placed in a container containing a liquid and a generating media such as Al.sub.2 O.sub.3 or ZrO.sub.2. The container is rotated for several hours to reduce the particle size. The resulting powders, however, are jagged, which makes them hard to handle, pack, and sinter. The flow characteristics of the resulting powders are particularly affected. The powders also are often contaminated with pieces of the milling media or container.
The ideal particles for manufacturing a strong ceramic structure, for example, a nose cone for a missile, are spherical and have a fine grain size ranging from 0.1 to 10 .mu.m. The particles preferably do not have a uniform size. Rather, to provide better packing, a distribution of particle sizes is preferred. By way of illustration, visualize a large box. Regardless whether the box is filled with basketballs, baseballs, or marbles, the pore space (the empty space between the spheres) will be the same. This is true because pore volume is a function of geometry, not size. If, however, each of the three types of objects were mixed in the same box, the pore volume would be greatly decreased. This is because the baseballs would occupy the spaces between the basketballs, and the marbles would occupy the spaces left between the other two. This is a more efficient packing method. Occasionally, voids may be desirable, as in the case of thermal insulation. For wear and strength applications, however, higher density is generally more desirable. Higher purity powders typically produce higher purity ceramic structures.
Spray processes such as flame plasma spraying, melt and then resolidify powders. The particles are generally spherical. Their size depends on many factors, including rheology, cooling, and precipitation rate. Neither of the concepts of using combustion to synthesize materials or using thermal spray techniques to manufacture ceramic powders is new. In 1968, the National Academy of Sciences assigned the Materials Advisory Board of the Academy the task of exploring solutions to problems faced by the Department of Defense. The committee performing the study recommended exploring the use of molten-particle or thermal spray processes for: the manufacture of fine, spherical ceramic powders; the production of high-purity oxide "smoke" by burning high-purity metals in oxygen; and the manufacture of uniform, reliable, free-standing structural ceramics. The Committee on Ceramic Processing, Materials Advisory Board, Division of Engineering, National Research Council, "Ceramic Processing" Publication #1576, National Academy of Sciences, Washington, D.C., 1968 ("NAS Report"). Spherical ceramic powders are highly desirable for thermal spray coatings and for the manufacture of solid, free-standing structures. Spherical particles lack surface irregularities which can cause snagging and agglomeration, one of the biggest problems in handling fine ceramic powders.
Since the NAS Report was issued, there has been much research into the use of spray processes and/or combustion to produce ceramic materials. Many of the spray processes are known as "aerosol processes." These typically involve combustion, which is used to drive a chemical reaction, rather than to directly produce a material. Currently, there are several different approaches to the production of ceramic powders using aerosol processes. These processes include flame, plasma, condensation, reaction, and spray pyrolysis processes. Aerosol spray processes such as flame and plasma are fairly mature technologies. Their primary use is not to manufacture powders, but to deposit coatings from pre-cursor powders. For example, a tungsten carbide coating is manufactured from tungsten carbide powder. (These coatings typically are used for wear and thermal barrier applications.) In general, these processes each involve the injection of a precursor feed material into a flame. The powders react, vaporize, and then condense to produce a ceramic powder. The features of these processes are given in Table I.
TABLE I ______________________________________ Typical Characteristics of Powders Produced by Aerosol Processes Process Particle Diameter Particle Shape ______________________________________ Flame 0.001 to 0.5 .mu.m Aggregates Plasma 0.001-1 .mu.m Spherical or Aggregates Condensation 0.001 to -10 .mu.m Spherical Reaction 0.001 to -10 .mu.m Spherical or Aggregates Spray Pyrolysis 0.1 to 10 .mu.m Spherical or Broken Shells ______________________________________
The commercialization of these processes has been limited. Even though they may be technically feasible, little was known about how they actually worked. This lack of understanding made the design of large scale production facilities difficult. Some of these gaps have since been filled through systematic study. Nonetheless, the results yielded on a small laboratory scale arc often difficult, if not impossible, to extrapolate to a full-scale production facility.
One alternative ceramic processing technique is self-propagating, high-temperature synthesis ("SHS"). SHS reactions usually involve the combustion of two solid phase materials. For example, to manufacture titanium boride (TiB.sub.2), titanium and boron are mixed and burned. The reaction is described by: Ti+2B=&gt;TiB.sub.2. The resulting combustion reaction is highly exothermic, that is, once the components start reacting, a large amount of heat is released which keeps the reaction going (hence, the designation self-propagating). The preparation rate of an SHS reaction, however, is relatively slow.
Engineering materials are often called upon to perform in particularly harsh environments. For example, aerospace applications have extremely demanding technical requirements. High G loading and thermal heating require strong, lightweight, environmentally resistant structures. Currently, there is no shortage of candidate high performance materials that can be used to manufacture these structures. Superalloys, ceramics, composites, and cermet materials can be used at temperatures up to several thousand degrees Kelvin. The problem in selecting between these materials in these applications is not satisfactory technical performance but, rather, reasonable cost. These materials can be stronger and lighter than conventional aerospace materials when processed correctly and can be used at much higher temperatures. Unfortunately, their price is often prohibitive. For example, 1 micron, 99.9% silicon nitride powder can cost $77 per 100 g.
Several prior patents disclose the production of certain types of coatings or the use of various combustion processes to apply coating materials. Nevgod, et al., U.S. Pat. No. 4,669,658 (Jun. 2, 1987) for GAS DETONATION COATING APPARATUS, is directed to a coating apparatus. The invention comprises a barrel enclosed in a casing, a spark plug associated with the barrel, a gas supply system, a buffer unit, and a powder supply system. A combustible gas is detonated which forces the powder through a muzzle at the end of the barrel in order to coat a substrate. No synthesis is accomplished. This is merely a coating process. Another patent to Nevgod, et al., U.S. Pat. No. 4,687,135 (Aug. 18, 1987) for DETONATION-GAS APPARATUS FOR APPLYING COATINGS, is directed to another coating apparatus. The invention lies in a specific gas distribution system.
Jackson, U.S. Pat. No. 4,902,539 for FUEL-OXIDANT MIXTURE FOR DETONATION GUN FLAME-PLATING issued on Feb. 20, 1990 is directed to a process for flame plating which uses a particularized fuel-oxidant mixture. Similarly of interest in this regard is a patent to Ulyanitsky, et al., U.S. Pat. No. 5,052,619 for BARREL OF AN APPARATUS FOR APPLYING COATINGS BY GAS DETONATION, issued on Oct. 1, 1991, is directed to a coating apparatus. As is the case with other prior art inventions, this patent is directed to coating, and not synthesis of materials.
Other patents were directed to material synthesis. For example, Taylor, U.S. Pat. No. 5,073,433 for THERMAL BARRIER COATING FOR SUBSTRATES AND PROCESS FOR PRODUCING IT, issued Dec. 17, 1991 is directed to a stabilized zirconia coating material and a process for making the same. Both Sue, et al., U.S. Pat. No. 5,185,211 for NON-STOICHIOMETRIC TITANIUM NITRIDE COATING, issued Feb. 9, 1993 and Sue, et al., U.S. Pat. No. 5,242,753 for SUBSTOIHIOMETRIC ZIRCONIUM NITRIDE COATING, issued Sep. 7, 1993 are directed to a particularized coating materials.
In spite of the existence of these prior materials synthesis and application processes, there remains a significant need for a commercially feasible method for synthesizing and applying engineering materials, particularly in high performance applications. Particularly, in applications which require extreme performance, there continues to be a substantial need for simple, reliable, and low-cost material synthesis and application techniques.